xen-bus / xen-block: add support for event channel polling

This patch introduces a poll callback for event channel fd-s and uses
this to invoke the channel callback function.

To properly support polling, it is necessary for the event channel callback
function to return a boolean saying whether it has done any useful work or
not. Thus xen_block_dataplane_event() is modified to directly invoke
xen_block_handle_requests() and the latter only returns true if it actually
processes any requests. This also means that the call to qemu_bh_schedule()
is moved into xen_block_complete_aio(), which is more intuitive since the
only reason for doing a deferred poll of the shared ring should be because
there were previously insufficient resources to fully complete a previous
poll.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20190408151617.13025-4-paul.durrant@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

1 files changed, 9 insertions, 2 deletions

diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
index 2210526490..7503eea9e9 100644
--- a/hw/xen/xen-bus.c
+++ b/hw/xen/xen-bus.c
@@ -932,13 +932,20 @@ struct XenEventChannel {
     void *opaque;
 };
 
+static bool xen_device_poll(void *opaque)
+{
+    XenEventChannel *channel = opaque;
+
+    return channel->handler(channel->opaque);
+}
+
 static void xen_device_event(void *opaque)
 {
     XenEventChannel *channel = opaque;
     unsigned long port = xenevtchn_pending(channel->xeh);
 
     if (port == channel->local_port) {
-        channel->handler(channel->opaque);
+        xen_device_poll(channel);
 
         xenevtchn_unmask(channel->xeh, port);
     }
@@ -973,7 +980,7 @@ XenEventChannel *xen_device_bind_event_channel(XenDevice *xendev,
 
     channel->ctx = ctx;
     aio_set_fd_handler(channel->ctx, xenevtchn_fd(channel->xeh), true,
-                       xen_device_event, NULL, NULL, channel);
+                       xen_device_event, NULL, xen_device_poll, channel);
 
     QLIST_INSERT_HEAD(&xendev->event_channels, channel, list);